1. Field of the Invention
Embodiments of the present invention relate to an electrically actuated friction brake with a brake pad actuated by an actuation device.
Embodiments of the present invention relate to electrically actuated brakes, thus to brakes in which an electrical actuator such as an electric motor, via transmission parts such as levers, screws, ball screws, cams, eccentrics, fluids, gases, etc., presses on the brake pad, such as a brake disc for example, on the friction surface, such as a brake disc or brake drum, for example. The design of the force characteristics over the actuation travel in electrically actuated brakes is important for the actuation time and the energy expenditure for applying the braking torque.
2. Discussion of Background Information
Especially for electrically actuated service brakes of vehicles, there are high standards with regard to short actuation times and pressing force requirement. For example, today's vehicles demand an actuation time for full braking of around 200 ms. At the same time, in modern vehicle front wheel disc brakes, brake pad pressing forces of 30 to 40 kN can arise, in many cases even significantly more. Since actuation travel*pad pressing force is the energy requirement for actuation of the brake and leads to the required actuation power for a given actuation time, it is apparent that the electrical actuators require accordingly high levels of electrical power. If an actuation travel of 2 mm is covered for full braking for 40 kN of pad pressing force, the energy requirement is roughly 40 Ws. If the braking process here takes 0.2 s, an average mechanical power of at least 200 W per brake is required, which must be provided by the electrical actuator. Available mounting space, weight, costs, and current requirement for the electrical actuator require that the motor power be kept low, which is why an optionally large electrical actuator cannot be used.
For a linear electrically actuated brake, i.e., for linear transmission elements such as screws, ball screws, and fluids with a linear relationship between the actuation travel and actuator (force, torque), a pad pressing force rising linearly from zero to a maximal value is necessary, under the assumption of a constant coefficient of friction. The necessary transmission ratio of the linear brake is determined here by the required maximal force (full braking), as this must be guaranteed, and remains constant for all lower pad pressing forces. That is disadvantageous, however, because, in all other, generally more frequent cases, the electrical actuator cannot be optimally utilized and is over-dimensioned. For such a linear brake, the electrical actuator is thus operated, up to design full braking, with a smaller than possible load, while the transmission ratio and thus also the attainable actuation time are determined by the constantly high transmission specified for the case of full braking. As a result, optimal and/or short-as-possible actuation times cannot be achieved for linear brakes for braking that does not correspond to the case of full braking.
In addition, the cost pressure on electrically actuated brakes is also high, because they have to compete with relatively simple hydraulic brakes. Therefore, any possible cost optimization of the electrical actuator is important. It is understood here that the smaller the electrical actuator can be kept, the more advantageous it will be.
Non-linear electrically actuated brakes such as those described in WO 2010/133463 A1, in which a non-linear transmission element, such as a cam, eccentric, non-linear ramp, etc., is provided between the actuator and brake pad, offer an improvement over linear brakes. In WO 2010/133463 A1, for example, a shaft with an eccentric pin, or a cam to which the brake pad is secured, is turned by an actuation device. Here the torque of an electric motor is transmitted via a linkage and lever to the non-linear transmission element of the brake. Due to the eccentricity of the pin or cam, the brake pad is pressed against the friction surface, and a non-linear relationship arises between the actuation travel or angle of rotation of the shaft and the pad pressing force or the arising braking torque. Due to the eccentric or cam, a force transmission also arises (a small travel effects a high force), whereby the electrical actuator can be dimensioned so as to be smaller. This also makes it possible to shorten the actuation times in comparison with a linear electrically actuated brake.
As a rule, the installation conditions of the brake, in particular for vehicle brakes, are such that only a very limited mounting space is available to receive the brake, so that electrical motors of small size have to be used. The very high pressing force of the brake pad must be produced from the high speed of the preferably small electric motor. Instead of the linkage and lever of WO 2010/133463 A1, this can be also be achieved, for example, by a transmission driven by an electric motor. For example, the output stage of the transmission rotates the shaft that is integrated in the transmission and has the eccentric or cam, while the non-linear transmission element again acts on the brake pad. With such a transmission, even transmissions of 1:40 can be implemented in a tiny mounting space, whereby small electric motors can be used. The actuation time can thus be reduced even further. But such transmissions are very complex and therefore also expensive.
A parking brake is known from WO 01/90595 A1 in which a brake actuating linkage is actuated by an electrically driven drive connection. The drive connection is embodied in the form of a cam disc that is rotated by the electric motor and an adjusting element guided along a surface of the cam disc. The cam disc can be embodied such that a constant torque is set on the electric motor in order to shorten the braking time and to achieve a particularly rapid translational movement of the brake actuating linkage.
For release of a friction brake, often a return spring is tensioned, which is released during release of the friction brake, and opens the friction brake by the energy released thereby. For example, DE 10 2006 012 076 A1 shows an electrically actuated friction brake in which a return spring is tensioned during actuation, and is released for release. The electrical drive must therefore supply energy for tensioning the release spring during the entire actuation of the friction brake.